3

Maybe a boil:
Some people go through phases where they get recurrent staphylococcal infections. It may be helpful to see the doctor when it is ready to pop to get a culture. There is a prescription bactroban that can be very helpful for such infections but you need to be seen to get it.
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4

Abscess:
You may have skin infection going on. The pimple has spread inflammation around and that's what causes pain and swelling. See doc to get it treated. Don't tamper with it yourself.
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6

Pimple:
Unfortunately we cannot dispense any advice to anyone below 16 and since you didn't mention your age, I must assume you are a young minor. Please see your parents and your doctor for your concern.
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8

Eczema vs. Psoriasis:
Raised defined circular red bumps on skin could be a form of psoriasis or a type of eczema. You need to see a dermatologist or podistrist for evaluation and treatment. Your doctor may perform bloodwork on you to see if you have any signs of auto-immune disease and/or they may prescribe oral and/or topical medications to help resolve this skin issue.
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9

Possibly boils:
Hard, painful pimples suggest a localized skin infection usually caused by the bacteria staph aureus. A possible reason it is behind the knee is that your grandson may have eczema which is often behind the knee and makes one more susceptible to staph infections.
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10

Rashes:
Are best evaluated by being seen...Literally. Blister like pimples on elbows and knees could be molluscum conatgiosum...Can't tell without seeing. You don't say how long they have been there. Did they appear since he was on the medication or before? Could be a viral rash. Does not sound like a typical allergy rash from medication..Make an appointment with your physician for the best answer.
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